1. Field of the Invention
The present invention relates to optical spectroscopy, and in particular probing optical spectral properties of a target material or device using a probing optical beam with multiple frequency swept sidebands to determine the optical spectral properties of the target. Further the present invention relates to temporal mapping of the optical spectral features of a target material or device, achieving kilohertz (103 Hertz) resolution over many GigaHertz (109 Hertz) of bandwidth about an optical center frequency. As used herein, optical spectral properties refer to the frequency dependence of a physical property (such as absorption, reflection or resonance, among others) of a target in the optical frequency range from about ultraviolet (1016 Hertz) to infrared (1012 Hertz)
2. Description of the Related Art
Optical spectral properties of various materials and devices (called optical targets herein) are important in commerce. For example, the optical spectral properties of laser tuning cavities and materials are often determinative of the applications for which a laser can be deployed. In emerging technological fields, such as described in Merkel II, information is programmed into the optical absorption spectra of certain materials, such as inhomogeneously broadened transition (IBT) materials also called spatial-spectral (S2) materials or S2 holographic materials; and retrieving the information involves detecting the optical spectral properties of the programmed material. Such programmed materials offer the capacity to process high time-bandwidth product signals more accurately and quickly than existing methods, as described in Merkel II. High time-bandwidth product signals occur in a broad range of fields, from real-time spectral analysis and medical imaging, to optical ranging and communications, to photonic analog-to-digital conversion, to high resolution RADAR and LIDAR applications, among others.
One approach to detecting the optical spectral properties of a target is to probe the target with an optical beam that sweeps through a range of frequencies, a so-called optical chirp in analogy to the sound made by an acoustic signal that sweeps through a range of audible acoustic frequencies. The optical chirp may be constant in amplitude and linear in frequency with time or may be modulated in amplitude and non-linear in frequency over time. The measured temporal response of the target to the chirped probe beam gives an indication of the optical spectral content of the target.
Chang I and the journal article, Chang et al, Physical Review A, 70 063803 (2004), entitled “Frequency-chirped readout of spatial-spectral absorption features” (hereinafter referenced as Chang II, the entire contents of which are hereby incorporated by reference as if fully set forth herein) describe how mapping spectral absorption features into temporal intensity modulation using a chirped optical field depends on the chirp rate of the field. When probing an arbitrarily complex spatial-spectral grating with a chirped field, a beat signal representing the grating period can be created by interfering the emitted photon echo chirped field with a reference chirped field, regardless of the chirp rate.
In previous approaches, the probe optical beam has been a frequency chirp of the primary optical carrier. While suitable for many purposes, there can be disadvantages to this approach. For example, an acousto-optical modulator (AOM) may be used to create such a chirp, however these devices are limited in their chirping bandwidth to approximately 1 GHz. Another approach is to utilize a chirped external cavity diode laser (CECDL), which has been shown to chirp over wide bandwidth, however these devices do not currently offer sufficient inherent frequency stability of the chirped optical carrier, thus eliminating their capability of discriminating fine features of the target optical spectrum.
In another approach, described by Patent Cooperation Treaty (PCT) Application Serial No. PCT/US2004/014019, filed May 6, 2004 entitled “Method and Apparatus for Optical Broadband Frequency Chirp” (hereinafter referenced as Harris), an attempt is made to splice together multiple limited band chirps in an optical ring in order to produce a chirp with greater bandwidth. While suitable for some purposes, this approach can introduce phase mismatches at overlapping frequencies and add complexity to the process of detecting the optical spectral properties of a target.
Another approach is described by U.S. Pat. No. 4,297,035 entitled “Method and device for detecting a specific spectral feature” (hereinafter referenced as Bjorklund). Bjorklund resolves a spectral feature from a target optical spectrum by modulating an RF tone or chirp onto a stable optical carrier and detecting the RF modulation frequency. While suitable for some purposes, this approach suffers from a requirement of utilizing photo detectors and digitizers with an equivalent bandwidth to the RF chirp or tone that is applied to the optical carrier. As described in Merkel I, it is well known that increasing the bandwidth of photo detectors and digitizers corresponds to an increasing noise floor of the device, thus making this approach impractical for analysis of broad band spectral features of interest here.
It is clear from the preceding description that there is a need for techniques that probe the optical spectral properties of targets without suffering one or more of the disadvantages of the prior approaches. In particular, there is a need for techniques to perform spectral-to-temporal mapping with high resolution over large bandwidths.